the extra sert of connectors with multi coloured blocks is a blaupunkt thing i think. on the punto mk2 the yellow block is used to check the security code of original radio and when you put in after market one you do not connect them wires up. also had feeling it wasnt fiat going by wire colors.
does yoru standard radio have a wiring diagram on it if so try and comapre it with one wich should be in back of car stereo manual make sure the permanet live and switchable live are macthed up.
but first give it atry with yellow plug removed. i know in unto if you put yellow plug in aftermarket one wierd things happen with sound when engine on.